1. Field of the Invention
The present invention relates to producing protective coatings on surfaces, and to manufacturing products such as lightweight lead-acid batteries therefrom.
2. Related Art
A coating generally refers to a relatively thin layer of a material that is deposited/laid on a relatively thick piece (usually) of a different material (often referred to as the substrate) in such a way that the coating adheres well to the substrate. Common examples include, but not limited to, coating of steel with oxides such as aluminum oxide for protection against corrosion and coating of aircraft components with thermal barrier materials such as zirconium oxide. Coatings may be relatively thick i.e., tens of micrometers in thickness, or relatively thin i.e., a few micrometers in thickness or even less. Aluminum oxide coating and zirconium oxide coating, noted above, respectively represent a relatively thin coating and a relatively thick coating.
A number of techniques have been developed to deposit coatings of a wide range of materials—metals, metal compounds (including oxides and nitrides), semiconductors, insulators, and polymers. These may be broadly classified as (a) physical and (b) chemical techniques. The physical techniques include thermal and e-beam evaporation, dc/rf/magnetron sputtering, ion plating, cathode arc deposition, and plasma spraying. Electrochemical deposition, chemical vapor deposition, dip coating, and spray pyrolysis are some of the chemical techniques for coating formation.
The physical and chemical techniques noted above are generally employed in industry for the manufacture of a variety of goods and articles in common use. In many of these methods, the substrate to be coated is maintained at ordinary room temperature, enabling the coatings to be applied to even those substrates (such as organic polymers), which cannot usually withstand high temperatures. However, chemical methods (save electrochemical deposition) require that the substrate be raised to an elevated temperature in order that chemical reactions that lead to the formation of the desired coating occur at a rate high enough to be practically and economically viable. Indeed, as is well known, at least some chemical reactions do not take place at a measurable rate unless the temperature is sufficiently high.
In general, the rate of deposition of coatings (measured, say, in micrometers per hour) through physical methods is lower than through chemical methods. An exception is the plasma spray method, in which the temperature of the surface to be coated is raised to high levels (thousands of degrees Celsius) at the points of coating. The high temperature enables the coating to be carried out at a high rate and aids the adherence of the coating to the substrate. However, such a process may be unsuitable for substrates with low melting points.
Thus, it may be appreciated that chemical methods (and the aforementioned physical method) offer higher deposition rates, but generally require temperatures which may be too high for certain applications. Accordingly, it is desirable to have a coating method that offers the advantages (e.g., high rate and cost-effective deposition) of chemical methods, but yet leaves the substrate intact at the end of the process, despite the elevation of temperature that might be necessary.
Such a coating process may be desirable in manufacturing several types of devices. An example of such a device is a lead-acid battery, with enhanced energy density, as described below.
A battery refers to a device which stores electrical energy such that the energy is available at desired times on demand. Batteries typically contain two electrodes, i.e., an anode (negative plate) and a cathode (positive plate) placed in an electrolyte. Electrical energy is generally obtained from a battery as and when desired by connecting the electrical appliance to be operated to its negative and positive plates, as is well known in the relevant arts.
Lead-acid batteries are in widespread use in several places such as automobiles, boats, airplanes, and for emergency power supply (uninterrupted power supply). In one conventional design, the negative and positive plates/grids of lead-acid batteries are implemented using lead alloys having lead in abundant proportions (including pure lead). At these electrodes, a primary role of lead/lead alloy is to provide a path for electrical conduction during battery charging and discharging.
Although lead alloys are not generally as good electrical conductors as metals such as copper, they are often preferred over other metals due to the stability they provide during battery charging and discharging, and their relatively low cost. Specifically, lead/lead alloys are typically able to withstand appreciably the highly corrosive environment created by the acid electrolyte. Metals more conductive than lead are either quickly corroded in acids (e.g., aluminium, copper) or too expensive to be commercially viable (e.g., platinum).
In lead-acid batteries, the framework of a battery plate that supports the active material and also serves as the current collector is referred to as the “grid”. In the battery terminology, the plate is also called an electrode. By definition, the electrode is an electronic conductor, which acts as a source or a sink of electrons involved in electrochemical reactions taking place in such a battery.
While plates/grids made of lead/lead alloys are reasonably stable in the acidic environment of the lead-acid battery, they are nevertheless corroded during the normal operation of the battery, limiting the lifetime of such a battery. It is thus desirable to provide protection against such corrosion, so as to enhance the durability of lead-acid batteries.
Furthermore, due to the high mass-density of lead (11.3 grams per cubic centimeter), lead-acid batteries are usually heavy. It is often desirable that batteries store high energy, but weigh less. Accordingly, a metric referred to as energy density, which is measured by the number of watt-hours (Wh) of energy stored in a battery per kilogram weight (kg) of the battery (abbreviated as Wh/kg), is often used to measure the efficacy or desirability of a battery. Thus, it is generally desirable to provide batteries with high-energy density.
For example, electric vehicles (EVs) for neighborhood applications such as hospitals, industrial parks, holiday resorts, residential communities, and city centers require batteries with high-energy density because, otherwise, the traction of the heavy batteries would in itself consume a sizeable fraction of the stored energy of the batteries. It is estimated that, for such applications, batteries with energy density of 40-50 Wh/kg would be more appropriate.
By contrast, many lead-acid batteries currently available in the market have energy density of about 30 Wh/kg. High-energy density batteries are generally important also in portable power applications, e.g., airborne systems, in which the weight of each component/sub-system is typically at a premium. High-energy density lead-acid batteries would also be advantageous in conventional automobiles with internal combustion engines, as well as in hybrid EVs, where fuel efficiency would be marginally enhanced when batteries are lighter.
In attempting to achieve a significant reduction in the total weight of a lead-acid battery (without reducing the amount of energy stored), an effective approach, therefore, would be to reduce the weight of the plates/grids constituting the electrodes of the battery. This may be accomplished by replacing the electrode structure made entirely of lead/lead alloy with a structure that uses a lightweight material, which merely acts as a physical support (substrate), and is covered by a relatively thin lead alloy layer (or laminate) that performs the charge/discharge functions of the battery.
The use of such a composite structure for the battery plate/grid, instead of a relatively thick plate made entirely of lead alloy, can result in a significant reduction in the total weight of each plate/grid. A corresponding increase in the energy density of the lead-acid battery then ensues. However, the use of a relatively thin layer of lead increases the need for its protection against corrosion in the strong acid environment of the lead-acid battery. Some of such example approaches are briefly described below.
For example, U.S. Pat. No. 4,221,854, entitled, “Lightweight laminated grid for lead-acid storage batteries”, issued to Hammar et al. (hereafter Hammar) describes a lead-acid battery in which a grid/plate comprises a substrate made of a polymer (such as polyvinychloride) laminated with a thin lead/lead alloy foil. This combination reduces the weight of the battery plate/grid, contributing to an increase in the energy density of the battery. However, Hammar does not appear to describe a corrosion resistant coating (and/or a process for forming the same on the substrate). As a result, the plate/grids of Hammar may be subsceptible to corrosion in acid electrolytes, thereby limiting the durability of the corresponding batteries.
U.S. Pat. No. 4,713,306, entitled, “Battery Element and Battery Incorporating Doped Tin Oxide Coated Substrate” issued to Pinsky et al (hereafter “Pinsky”) describes a battery element useful as at least a portion (which appears to mean the grid) of the positive plate coated with electrically conductive doped tin oxide. The tin oxide coating does not appear to be meant to provide protection to the grid against acid corrosion since the grid made of glass fibre is inherently resistant to acid corrosion.
U.S. Pat. No. 5,643,696, entitled, “Battery plates with lightweight cores” issued to Rowlette describes battery plates/grids made of metallic substrates (aluminum or titanium or their alloys) coated with lead/lead alloy. These metallic substrates may still have unacceptably high mass density (e.g., aluminum has an approximate mass density of 2.7 grams cm−3). Accordingly, it may be desirable to produce batteries using substrates made of materials having an even lower mass density.
Another example approach is described in U.S. Pat. No. 6,232,017, entitled “Grid for lead-acid battery”, issued to Tsuchida et al (hereafter “Tsuchida”), in which polyamide and glass fibers are used to construct a composite battery plate/grid. The weight of the grid is reduced, in comparison with conventional grids made entirely of lead/lead alloys, by using the low density of polyamide to form a support structure, and a glass fiber sheet coated by a thin layer of lead/alloy to form the electricity-collecting part of the battery plate/grid. However, a corrosion resistant coating of the plates/grids appears to be absent in Tsuchida, just as it is absent in Hammar, thereby making the embodiments susceptible to acid corrosion.
In the U.S. Pat. No. 6,316,148, entitled, “Foil-encapsulated, lightweight, high-energy electrodes for lead-acid batteries”, Timmons et al (hereafter Timmons) describe another approach to reduce the weight of lead-acid batteries. The electrodes are made of non-lead substrates (such as aluminum) encapsulated by thin sheets of conductive foils of lead/lead alloy, which conduct electricity. The foils, being corrosion-resistant, protect the substrate from acid corrosion. The weight of the battery is reduced by the use of non-lead substrates with mass density no greater than 70% of the mass density of lead. However, Timmons also appears to suffer from the same inadequacies of Hammar and Tsuchida in that a corrosion-resistant coating of the plate/grid appears to be absent.
In addition to providing high-energy density, it may be desirable to produce batteries meeting several other requirements. For example, it may be desirable to use cost-effective material for the substrates to reduce the overall cost of producing batteries. The overall manufacturing technology may further need to allow thin coatings of lead alloy on such a cost-effective and lightweight (low mass density) substrate material. The technology may further need to allow corrosion resistant coatings to be applied to the plates/grids, in order to enhance the durability of the resulting lead-acid batteries.
One problem with the use of a low cost material, which also has a low mass density as the substrate is that the melting point of such a material may be low, making it incompatible with several technologies employed in the manufacture of lead-acid batteries. Similar incompatibility may also exist when applying a corrosion resistant (yet with a sufficiently good electrical conductivity) coating on the lead alloy layer.
For example, the formation of such a corrosion-resistant coating (as that of tin oxide) on the lead/lead alloy layer of the battery plate/grid usually requires a temperature significantly higher than 327° C., the melting point of lead. (The melting point of lead alloys usually employed in lead-acid batteries is lower than 327° C.) In particular, the formation of a tin oxide layer by the simple and convenient “dip coating” method requires calcination at a temperature in the range 450-600° C., as described in the article entitled, “Development of positive electrodes with SnO2 coating by applying a sputtering technique for lead-acid batteries”, by Kurisawa et al., published in the Journal of Power Sources 95 (2001) pp. 125-129 (hereafter “Kurisawa”).
Accordingly, it is stated in the abstract of Kurisawa that, “ . . . it is impossible to apply this (dip coating) method to a Pb (lead) substrate . . . ”, appearing to imply that the calcination temperature required to form the protective tin oxide layer in this method is incompatible with the lead/lead alloy plate/grid.
Attempts have therefore been made by Kurisawa to use coating technologies that employ low temperatures. For example, Kurisawa describes using vacuum-based thin film technology to form protective SnO2 coatings on lead grids. In such a coating process, the temperature of the lead grid to be coated does not exceed approximately 120° C. Specifically, Kurisawa teaches using the radio frequency (RF) sputtering technique to deposit a 15 micrometer thick SnO2 coating onto a 500 micrometer-thick lead plate. Such a protective coating of SnO2, on the relatively thin lead plate/grid, has been shown in Kurisawa to be effective in reducing positive plate/grid corrosion in lead-acid batteries and in improving the energy density of the batteries thereby.
However, sputtering is generally a slow process (e.g., Kurisawa indicates a rate of 0.4 micrometer/hour), which may require that the substrate (along with the lead alloy coating) be subjected to ambient temperatures (e.g., 120° C. in Kurisawa) for many hours. The prolonged exposure to such ambient temperatures may compromise the mechanical integrity of the plates/grids having low melting points, even if the ambient temperatures are lower than the melting point of the materials forming the substrate and the coatings.
Furthermore, as sputtering is generally a line-of-sight deposition process, both sides of an electrode (plate/grid) cannot be coated in a single step, unless complex and expensive sputtering apparatus is employed. The cost of forming a corrosion-resistant coating on a battery plate/grid by such a sputtering process is likely to be high, which may not be acceptable in several applications.
Therefore, what is also required is a process, which allows the corrosion-resistant oxide coating of lead/lead alloy layers to be formed on battery plates/grids constructed from a lightweight, inexpensive material, usually having a low melting point. Such plates/grids may then be used to fabricate lead-acid batteries, which would have a higher energy density and a longer lifetime than the conventional lead-acid batteries.